Does Undertitan's +2 ability count as a trigger?

Yes, Undertitan’s ability "When you play me, give your other units +2 to all units you control this turn" is a triggered ability.

According to Rule 376.1, triggered abilities are recognized by the word "when" followed by a game action or event. Because Undertitan's ability begins with "When you play me," it meets the definition of a triggered ability.

Regarding the mechanics of this ability:

• Triggered Ability: As defined in Rule 376.3, when the condition is met (in this case, playing the unit), the ability is placed on the chain.
• Effect: The ability gives your other units +2 to all units you control this turn.
• Chain Resolution: While Rule 376.3 describes how triggered abilities are placed on the chain, the Last-In-First-Out (LIFO) resolution order is a fundamental mechanic of the game's chain system, as outlined in Rule 336.1, which states "The newest item on the Chain resolves."

Note: I could not fully verify this ruling against the retrieved rules.